This invention relates to rechargeable battery cells in which ions of a source electrode material move between cell electrodes through an intermediate electrolyte during the charge and discharge cycles of the cell. More particularly it is concerned with a crosslinked solid polymer electrolyte which terpolymer network plus salt plus a plasticizer constitutes the polymer electrolyte that is ionically conductive. The solid polymer electrolyte is formed by first dissolving an electrolyte salt in a solution including a combination of selected monomers together with a plasticizer and then spreading the solution into a thin layer whereupon the layer is heated or otherwise subjected to a source of energy to effect its polymerization. Any one of the resulting solid polymer electrolytes is well adapted to be used in solid state batteries, supercapacitors, fuel cells, sensors, electrochromic devices and the like.
Solid polymer electrolytes have been proposed in the past for use in place of liquid electrolytes in such equipment because they combine in one material the function of electrolyte, separator, and binder for the electrode materials, thereby reducing the complexity of the ultimate structure. The advantages inherent in the use of a solid polymer electrolyte are the elimination of possible leakage and it preclude the possibility of dangerous increases in pressure which sometimes occur when volatile liquid electrolytes are present. Further such solid polymer electrolytes can be fabricated as thin films which permit space efficient batteries to be designed. Also flexible solid polymer electrolytes can be fabricated which allow for volume changes in the electrochemical cell without physical degradation of the interfacial contacts.
A number of solid polymer electrolytes have been suggested for use in the prior art such as thin films formed by complexation between lithium salt and linear polyether for example poly(ethylene oxide) and poly(propylene oxide). Although these solid polymer electrolytes do have some significant properties such as high electrochemical and chemical stability characteristics as well as ease of fabrication in the form of thin films, they have not met with any appreciable commercial success because the conductivity of such electrolytes at ambient temperatures is poor. The need to restrict the use of such electrolytes in electrochemical devices at elevated temperatures clearly limits the number of possible useful applications.
Various attempts have been made to improve the ionic conductivity of polymer electrolytes by a selection of new polymeric materials such as cation conductive phosphazene and siloxane polymers. Other suggestions include the use of the addition of plasticizers to polymer electrolytes to form "wet" polymer or "gel electrolyte" which procedure does improve ambient temperature conductivity but this is done at the expense of mechanical properties. So to date no commercially useful solid polymer electrolyte has been developed in the form of a thin film that has good mechanical properties and ionic conductivity in the range of .gtoreq.10.sup.-3 S/cm at ambient temperatures as well as enhanced electrochemical stability for use in, for example, a high energy-rechargeable solid state battery or for other applications in electrochemical units in which high ionic conductivity at ambient temperatures is a requirement.